The transmission of soliton pulses or "solitons" is a known phenomenon. These pulses are return-to-zero (RZ) pulses of time width (full width at half maximum or FWHM) that is small compared with the bit time, that present a determined relationship between power, spectrum width, and time width, and that generally propagate in that portion of an optical fiber which has abnormal dispersion. The way the envelope of such a soliton pulse varies in a monomode fiber can be modelled using the non-linear Schrodinger equation; propagation relies on equilibrium between fiber dispersion and fiber non-linearity.
The transmission of such pulses is limited by various effects such as jitter induced by solitons interacting with the noise present in the transmission system, as described for example in the article by J. P. Gordon and H. A. Haus, published in Optical Letters, Vol. 1, No. 10, pp. 665-667. This effect, known as the Gordon-Haus effect, puts a theoretical limit on the quality or on the rate of soliton transmission. To exceed this limit, it is possible to make use of synchronous modulation of soliton signals by means of a clock signal or "clock" for the purpose of correcting their time jitter, as explained for example in an article by H. Kubota, published in IEEE Journal of Quantum Electronics, Vol. 29, No. 7 (1994), p. 2189 et seq.
To provide such synchronous modulation, it has been proposed to use the Kerr effect in synchronous phase modulators. Thus, the fiber itself can be used for phase modulation purposes. A presentation by S. Bigo, P. Brindel, and O. Leclerc at the Oct. 30, 1996 symposium on guided optics ("Journees nationales de l'Optique guidees") held at Nice (France) describes soliton signal regeneration by all-optical phase modulation. An optical clock is superposed on the soliton signal, thereby imparting a non-linear phase shift to the soliton signal pulses by copropagating with them in an optical fiber that includes a length which has been selected to minimize the effects of slip between the soliton signal and the optical clock. Reference may be made to an article by T. Widdowson et al., entitled "Soliton shepherding: all-optical active soliton control over global distance", published in IEE Electron. Letters, Vol. 30, No. 12, p. 990 (1994).
It has also been shown by S. Bigo, in a thesis, University of Bensancon, 1996 entitled "Traitement de signal tout-optique pour la transmission a tres haut debit de solitons par fibre optique" [All-optical signal processing for very high rate transmission of solitons by optical fiber] that an all-optical modulator using the Kerr effect, such as a non-linear optical loop mirror (NOLM) or a fiber, can be considered as a discrete sinusoidal modulator synchronized with the soliton train in spite of the slip or "walkoff" due to chromatic dispersion and to losses, providing the clock used is sinusoidal and the time offset between the signal to be modulated and the clock is appropriately adjusted.
One of the problems that arises with synchronous phase modulation is that of synchronizing phase between the clock and the soliton signal to be regenerated. In a conventional semiconductor modulator, such synchronization is conventionally achieved by deriving a signal whose intensity is representative of the phase difference between the modulating signal and the signals to be modulated. Feedback is then used to adjust the phase difference. Nevertheless, that solution is not applicable to distributed optical phase modulator devices using the Kerr effect in which there is no signal available of intensity that enables the phase of the modulator signal to be determined.